1. Field of the Invention
The disclosures herein relate to an imaging apparatus provided with a photoelectric conversion film and an electron source array having electron sources arranged in matrix form wherein electrons are emitted from the electron source array during a video signal output period and a vertical blanking period.
2. Description of the Related Art
Research has been conducted for some time with respect to imaging apparatus that is provided with a photoelectric conversion film and an electron emission array having a matrix of electron emission sources, from which electrons are drawn out by an electric field without application of heat. This electron emission array has a plurality of Spindt-type emitters arranged in matrix form, which are opposed to the photoelectric conversion film across vacuum space. In such imaging device, holes that are generated and accumulated in the photoelectric conversion film in response to light arriving from an external source are read out by using electrons successively emitted from the Spindt-type emitter array, thereby producing a time sequence of video signals (see Patent Document 1).
When highly bright light enters a portion of the photoelectric conversion film of such an imaging apparatus, a large amount of holes will be accumulated in this portion of the photoelectric conversion film on its electron scan side, resulting in a potential increase at this local portion. As electrons are successively emitted from the electron emission array, those electrons emitted toward the vicinity of the portion of the photoelectric conversion film that is illuminated by highly bright light have their trajectory bent towards this portion of the photoelectric conversion film having an extremely high potential. This phenomenon will hereinafter be referred to as “bending”. As a result of the bending, holes accumulated in this portion of the photoelectric conversion film will be read out. Consequently, the output video image suffers an artifact by which a highly bright object appears expanded from its original size (which will hereinafter be referred to as “blooming”), degrading image quality such as image resolution.
Further, when highly bright light enters a portion of the photoelectric conversion film, a large amount of holes are generated and accumulated in this portion. When this happens, it may not be possible to read out all the generated and accumulated holes by use of the electrons emitted from the electron emission array on a single scan, thereby creating a prominent capacitive residual image.
Moreover, the entry of highly bright light into the photoelectric conversion film causes a large amount of holes to be accumulated on the electron scan side of the photoelectric conversion film. This serves to drop the effective electric field applied to the photoelectric conversion film, resulting in a large amount of optically generated electric charge (electrons and holes) being trapped in the photoelectric conversion film. As the electric field in the photoelectric conversion film increases in response to scanning on the photoelectric conversion film by electrons successively emitted from the electron emission array, the optically generated electric charge is released from its trapped state. Holes of this freed electric charge are then accumulated on the electron scan side of the photoelectric conversion film. As a result, these holes that were trapped are read out on a next scan, creating a prominent photoconductive residual image.
In order to obviate these problems, research has been conducted with respect to a flat-type imaging device that continues to apply voltage to a gate electrode during a residual charge sweeping period that follows immediately after an image signal output period, during which a pixel signal for a horizontal scan line is read (see Patent Document 2, for example). The application of voltage serves to remove residual electric charge to prevent residual images from occurring in response to the entry of highly bright light.
The flat-type imaging device disclosed in Patent Document 2 also applies a voltage to the gate electrode of a next horizontal scan line in an excessively-accumulated charge sweeping period following the residual charge sweeping period, and sets the potential of cathode electrodes higher than the potential of a reference scan surface. This serves to remove the accumulated electric charge that is provided in excess of the amount readable within a video signal output period. With this arrangement, white saturation, smear, and resolution degradation resulting from imaging a highly bright object are prevented.
Patent Document 2 discloses dividing a no-video-signal-output period following a video signal output period into two periods including a residual charge sweeping period and an excessively-accumulated charge sweeping period. In the residual charge sweeping period, all the electron sources included in a horizontal scan line for which a video signal has just been output emit electrons to remove residual holes existing in the photoelectric conversion film at the position opposite to this horizontal scan line. The disclosed flat-type imaging device can thus prevent the occurrence of residual images responsive to highly bright incident light.
According to Patent Document 2, further, in the excessively-accumulated charge sweeping period, all the electron sources included in a horizontal scan line for which a video signal is about to be output emit electrons to remove excessive holes accumulated in the photoelectric conversion film at the position opposite to this horizontal scan line. The disclosed flat-type imaging device can thus prevent white saturation, smear, and the degradation of image resolution from occurring in response to highly bright incident light.
In Patent Document 2, however, electrons are emitted without exception during a no-video-signal-output period, i.e., during a blanking period, regardless of whether highly bright light enters the photoelectric conversion film. Because of this, electrons emitted during the blanking period are wasted if no highly bright light enters the photoelectric conversion film.
By the same token, electrons emitted during the blanking period toward a portion of the photoelectric conversion film where no highly bright light arrives are wasted even if highly bright light enters another portion of the photoelectric conversion film.
Such drive method ends up not only wasting all or part of emitted electrons but also imposing a heavy load on the electron emission array. This gives rise to a problem in that the reliability and product life of the electron emission array will be significantly degraded.
In Patent Document 2, further, electron sources included in a horizontal scan line for which a video signal has just been read and in a horizontal scan line for which a video signal is about to be read are made to emit electrons in a single blanking period during one field or frame period. When extremely bright light enters the photoelectric conversion film to accumulate a large amount of holes on the electron-scan side of the photoelectric conversion film, the amount of electrons emitted in a single blanking period fails to remove all the residual holes remaining in the photoelectric conversion film immediately following a corresponding scan and all the excessive holes accumulated in the photoelectric conversion film immediately prior to scan. There is thus a risk of suffering the occurrence of capacitive residual images and the degradation of image resolution due to blooming.
Further, the imaging device disclosed in Patent Document 2 cannot prevent the generation of a photoconductive residual image resulting from the lowering of an electric field within the photoelectric conversion film in response to highly bright incident light. Especially when a highly bright object in motion is filmed, the object leaves a visible trace on the screen. This causes a severe reduction in image quality.
Accordingly, there is a need for an imaging apparatus that can prevent the degradation of image resolution and the generation of capacitive or photoconductive residual images resulting from the entry of highly bright light into the photoelectric conversion film while reducing the load on the electron emission array.
[Patent Document 1] Japanese Patent Application Publication No. 6-176704.
[Patent Document 2] Japanese Patent Application Publication No. 2004-134144.